The stem of a cycle, e.g. a bicycle, connects the handlebar with the steerable front wheel through the fork; the fork holding the front wheel. The quill is the vertical portion of the stem. In many designs the quill and the stem are unitary. The stem and the quill are critical parts on a bicycle because all forces the rider's arms make while pedaling go through the stem and the quill to the frame, and if either of them breaks there is no way to hold the bicycle with the hands and the rider will consequently end up on the ground. Stems are often adjustable in different ways: Some or all of the height, the angle to a vertical plane, and the angle on a horizontal plane, may be adjustable in different combinations.
The setting for the long axis of the handlebar when the bicycle is being ridden is perpendicular to the plane of the front wheel. However, at the time of storing or transporting a bike on an elevator or a train, that setting may become inconvenient. Although this angle is usually adjustable, it requires a tool and takes significant time to adjust to and from the riding setting. In addition, the parts for accomplishing this adjustment are not designed to be used very often, e.g. any time it is desired to turn the handlebars parallel to the plane of the front wheel for storage or travel.
Foldable bicycles usually have foldable stems or quills while non-foldable bicycles typically come with handlebars that, at best, are only foldable with special tools and knowledge as well as substantial time and effort. Examples of patents disclosing foldable handlebars include U.S. Pat. Nos. 5,269,550; 5,419,573; 6,637,288 and 7,243,573.